1. Field of the Invention
The present invention relates to the manufacturing and testing of integrated circuits. More particularly, the present invention relates to electrical contacts on probe elements such as on membrane probe cards used in integrated circuit testing.
2. Description of Related Art
Integrated circuit chips are formed using fabrication processes which build many chips on wafers, which are for example 6 or 8 inches in diameter or larger. While the chips are still on the wafer in as-formed positions, a membrane probe can be used to electrically sense the performance of the circuits on the chips. See for example U.S. Pat. No. 5,180,977 entitled MEMBRANE PROBE CONTACT BUMP COMPLIANCY SYSTEM invented by Huff; U.S. Pat. No. 4, 980,673 entitled FORCE DELIVERY SYSTEM FOR IMPROVED PRECISION MEMBRANE PROBE invented by Huff et al.; U.S. Pat. No. 4,918,383 entitled MEMBRANE PROBE WITH AUTOMATIC CONTACT SCRUB ACTION invented by Huff et al; and U.S. Pat. No. 4,906,920 entitled SELF-LEVELING MEMBRANE PROBE invented by Huff et al. In accordance with this technology, costs savings are possible through early identification of faulty circuits before they are separated from the wafer, and thereafter assembled into chip packages.
Good electrical contact between the probe contacts and chip contacts, often called pads, is essential for accurate evaluation. The contacts of chips on the wafer are formed typically of aluminum or combinations of aluminum with other metals, and have a natural skin of hard aluminum oxide, or other natural hard skin caused by oxidation or other reactions with the environment. Wear of the probe contacts occurs as a result of repeated contact with the hard aluminum oxide skin, resulting in a degradation of the accuracy of the measurements over time.
In the prior art, the probe elements are formed with a contact bump technology usually based on nickel sulfamate plating to build up bump contacts. However the nickel deposit as a result of this plating technique is too soft for extended use, for example beyond about fifty thousand to one hundred thousand touchdowns. With such extended use, the contour of the nickel bump frequently develops an undesirable flattened area.
One prior art technique for improving the performance of the contact bumps involves creating a rough textured surface on the bump. See U.S. Pat. No. 5,487,999 entitled METHOD FOR FABRICATING A PENETRATION LIMITED CONTACT HAVING A ROUGH TEXTURED SURFACE invented by Farnworth. However, the roughening of the surface as described in the Farnworth patent does not overcome the problems of the softness of the nickel bump, while it may improve the ability to penetrate the hard skin on the contact pad.
Also, examination of the tips of worn nickel bump contacts at high magnification (scanning electron microscope) indicates that the soft nickel contact surface tends to accumulate aluminum oxide. The embedded aluminum oxide ultimately degrades performance by increasing the bump to pad contact resistance. Increased friction and mechanical resistance to sliding are also associated with contact wear and aluminum oxide pickup.
Accordingly, there is a need for a technique for extending the lifetime of the contact bumps used in membrane probe cards and in other testing situations where the contacts are required to make many thousands of contacts over their useful life.